1. Field of the Invention
The present invention relates generally to valve positioners such as are used in turbine systems. More particularly, the present invention relates to a method and corresponding apparatus for sensing a valve's position and detecting faults in the valve position sensing circuitry.
2. Description of the Prior Art
A conventional valve positioner (as used to, e.g., automatically control the flow of fuel or steam to a turbine) controls the movement of the valve by measuring the valve's position and comparing the measured valve position with a predefined setpoint. The valve is moved toward an open position if the measured position is less than the setpoint, and toward a closed position if the measured position is greater than the setpoint. This process is continued until the measured position is equal to the setpoint.
A problem with known positioners arises where a short or open circuit, due to, e.g., a loose or intermittent connector in the position sensing circuity, causes the measured position to always be less than the setpoint. This causes the positioner to continually move the valve toward its fully open position. Known methods of fault detection involve detecting an excessive mismatch between the measured valve position and the predefined setpoint. A disadvantage with these prior art methods is that they introduce a time delay to allow for sluggish actuator response. Thus, when a fault occurs (in, e.g., the position sensing circuitry) the setpoint must be reduced to prevent the positioner from running out of control. However, due to the above-mentioned time delay, these methods are slow in responding to the detected faults. Thus, for example, in a turbine system, the turbine could be damaged by an excessive flow of fuel or steam before a fault in the valve position sensor is detected, and the valve accordingly closed (or prevented from opening further).
It is therefore an object of the present invention to provide a simple and reliable means of immediately detecting faults in the valve position sensing circuit. In addition, the means employed should require a minimal amount of additional circuitry beyond that which is necessary to detect the valve's position. The present invention achieves these objectives.